Among image forming apparatuses configured in such a manner that a developing cartridge housing a toner is removably attached and in which the toner is fixed on a sheet of paper to form an image on the sheet of paper, an image forming apparatus proposed in Jpn. Pat. Appln. Laid-Open Publication No. 2000-221781 is provided with a photo interrupter in the housing section housing the developing cartridge, the photo interrupter being exposed in a manner of being opposite the developing cartridge.
Such image forming apparatus detects a protrusion of a gear provided in the developing cartridge by the photo interrupter in order-to allow the image forming apparatus to detect whether the developing cartridge is newly mounted in the image forming apparatus, and determines whether the developing cartridge is newly mounted in the image forming apparatus.